The polymorphic N-acetyl transferase enzyme found in the liver is responsible for metabolism of several pharmacological agents, including some medications that are used frequently in HIV positive patients. This study will determine whether the acetylator phenotype of the hepatic N- acetyl transferase enzyme is altered in 20 HIV infected patients with significant CD4 lymphocytopenia compared to 20 patients with CD4 counts of 500 or more. Additional research goals include the effect of ethnicity, nutritional status and co-infection with hepatitis B or C virus on acetylator phenotypes. The acetylator phenotype will be determined by measurement of 5-acetyl-6amino-3methyl uracil in urine collected for 6 hours after ingestion of caffeine.